Relatively small gas-liquid contacting columns of between 1 and 2 m diameter present a number of specific challenges for the design of an efficient gas-liquid contacting tray.
In a traditional design, a single segmental downcomer is arranged on each tray in a column. A segmental downcomer is a downcomer wherein the downcomer inlet opening is arranged near the wall so that part of the circumference of the downcomer inlet opening runs along the circumference of the tray (column wall). In a vertical gas/liquid contacting column, a plurality of such trays are stacked wherein consecutive trays are rotated by 180 degrees about the column axis, so that each tray receives liquid from the downcomer outlet of the next higher tray on a liquid receiving area diametrically opposite to the downcomer inlet opening. This design has however disadvantages.
One disadvantage is that the flow path length of liquid over the tray is relatively long, in the order of the diameter of the tray minus the width of the downcomer. Although this might be thought of as an advantage in the first place, it turns out that a relatively large gradient in liquid height develops on the tray during normal operation, between the liquid receiving area and the downcomer inlet opening. This maldistribution of liquid impairs tray efficiency and capacity in that gas preferably passes through gas passages in the area of low liquid height, near the downcomer inlet opening. Also, at the largest liquid heights, liquid can weep through the gas passages which limits tray capacity.
A further problem with single segmental downcomers is that in the flow pattern of liquid over a tray so-called dead zones are formed near the column wall, halfway between the receiving area and the downcomer inlet opening. The dead zones result in a lower tray efficiency, unless special measures are taken in order to improve the flow pattern.
A further disadvantage is that it is not possible to provide large downcomer inlet openings without compromising tray efficiency, in cases of high liquid load of the column. Liquid load can be expressed in terms of the flow parameter
      Φ    =                            V          1                          V          g                    ⁢                                    ρ            1                                ρ            g                                ,wherein the Vl and Vg are liquid and gas volumes in the feed per unit of time, and ρl and ρg are the densities of liquid and gas, respectively. At high liquid loads the flow parameter is equal to or larger than about 0.1.
In order to provide a large downcomer inlet opening, e.g. 20%-27% of the total cross-sectional area of the tray or more, a very wide segmental downcomer has to be used. However, such a downcomer still has a relatively low downcomer inlet length. The term downcomer inlet length is used in the description and in the claims to refer to the length of the circumference of the downcomer from which liquid can be received from the tray. This length is often provided with a weir in order to provide for a minimum liquid height on the tray. Therefore the downcomer inlet length is often also referred to as weir length even if no weir is arranged.
A relatively short downcomer inlet length in combination with a relatively large downcomer inlet area is undesirable because the inlet length becomes the limiting factor for liquid handling capacity. This results in relatively large liquid heights on the tray, which is generally unwanted since it contributes to premature jet-flooding, and therefore limits tray capacity.
An alternative tray design in smaller columns, in order to provide more downcomer inlet length, is the so-called two-pass tray. In this design two types of trays are used that are alternatingly stacked in a column. The first type of trays has two segmental downcomers that are arranged diametrically opposite to each other on the tray. The second type has a single rectangular downcomer along a diameter of the tray, which is arranged parallel to the segmental downcomers of the adjacent trays. The liquid flow path length in this two-pass design is in the order of half the tray diameter minus the downcomer width.
The two-pass tray design also has disadvantages. First, two significantly different types of trays have to be manufactured. Second, one tray type will normally be limiting, and it is nearly impossible to provide a fully balanced design. For example, the downcomer inlet length is significantly different on both tray types. Third, on the tray with the single diametrical downcomer there is normally no fluid communication between tray areas on either side of the downcomer, above and below the tray. Therefore different liquid levels may develop on both sides, and there is no vapour communication below the tray, and this impedes tray efficiency. In principle, fluid communication channels can be arranged between the two sides to alleviate the vapour communication problem, however this adds to complexity and cost of the tray.
In another tray design, which is often applied for larger trays, a plurality of parallel downcomers is arranged between the circumference of the tray and a virtual diametrical line. Examples of this tray design are disclosed in U.S. Pat. Nos. 6,460,833, 6,494,440, and 6,588,735. The arrangement of downcomers on the two tray sections is identical, such that one tray section can be transformed into the other by a rotation about 180 degrees about the centre of the tray. The total number of downcomers is even. On each tray section at least one substantially rectangular downcomer is arranged along a line perpendicular to the virtual diametrical line. Also on each tray section one segmental downcomer can be arranged in a corner between the virtual diametrical line and the tray circumference. The downcomers on the two tray sections form a staggered arrangement. Adjacent trays in a column are mirror images of each other with the virtual diametrical line as mirror.
This layout of downcomers on the tray works well for larger columns having a diameter above ca 2 m. It can also be applied for smaller diameter columns, however the design has to increasingly take account of a limitation that the flow path length, which is in the order of half the distance between adjacent downcomers on a tray section, does not become too small. This is in particular the case when a relatively large downcomer inlet area needs to be provided on the tray in order to provide sufficient liquid handling capacity. For new columns one could choose a larger diameter in order to provide a minimum flow path length, for retrofitting existing columns this is not possible.
For example, it can be calculated that, with one rectangular and one segmental downcomer per tray section in the known layout, a flow path length of say 250 mm parallel to the virtual diametrical line can only be realised on a tray with say 1.5 m diameter, if the total downcomer inlet area is less than 18% of the total cross-sectional area of the tray.